This invention relates to detecting apparatus for a welding gun to detect the degree of opening thereof, more specifically, to a detecting apparatus for a welding gun of the type in which a pair of arm holders provided for holding a pair of gun arms are pivotally supported on a gun bracket and the piston rod of a pressure cylinder attached to one of the arm holders is connected to the other arm holder so that the gun arms of the welding gun may be opened and closed by operation of the pressure cylinder and the degree of opening detected.
A conventional apparatus has been known as disclosed in the Japanese Unexamined Utility Model Registration Application Publication, Jikkai Sho 48-25014, in which a displacement detector is disposed between a pair of gun arms so as to detect the degree of opening from the relative displacement of the two gun arms with respect to each other.
In the foregoing type of opening-degree detecting apparatus, electric current is applied to the workpiece clamped between electrode tips attached to the front ends of a pair of gun arms of the welding gun. Thereafter, the amount of penetration into the workpiece of the electrode tips is measured from the degree of opening of the gun arms which is detected on the basis of output from the displacement detector in order to determine whether the amount of penetration is larger or smaller than the allowable value, whereby an abnormal or defective welding (such as holing) is detected.
In order to restrain spatter during welding from occurring, it is desired to control the welding current on the basis of the degree of opening of the welding gun, this degree of opening varying in accordance with the melting state of the workpiece. However, if a displacement detector is disposed between a pair of gun arms as in the foregoing material, the displacement detector is affected by the electromagnetic field produced by electric current flowing through the gun arms and therefore fails to operate normally when the gun arms are energized. For this reason, it is much desired to develop a detecting apparatus capable of accurately detecting the degree of opening of the welding gun even when the gun is energized.
In a welding gun which opens and closes a pair of gun arms thereof using a pressure cylinder attached to the arm holders, each gun arm is attached to each arm holder via an insulation material interposed therebetween or the pressure cylinder is attached to the arm holder via an insulation material interposed therebetween in order to prevent the gun arms from becoming shortcircuited when they are energized. Therefore, no current is flowing in the portion where the pressure cylinder is located, so that the intensity of electromagnetic field reaching the location of the pressure cylinder is much lower than that around the gun arms themselves.
In view of the foregoing, the present invention has for its object to provide a detecting apparatus capable of accurately detecting the degree of opening of the welding gun even when the gun is energized.